It is often desired to produce signal at a power level higher than can be produced by a single device. This is commonly done in audio power amplifiers, in which two (or more) tubes are combined in a parallel or in a push pull arrangement in order to double (or further increase) the output power available at a given distortion level. Push-pull operation is a form of paralleling in which harmonic distortion is cancelled, and is commonly used in high fidelity audio amplifiers and also in cable television (CATV) amplifiers.
Paralleling of multiple signal amplifier devices is very common at radio and microwave frequencies, because the physical limitations on the individual signal handling devices required by the high frequencies of operation tend to reduce the power-handling capability of the individual signal handling device. A large body of art is directed to arrangements for combining the output power of a multiplicity of microwave signal handling devices such as amplifiers. Some of these are described in the article "Miorowave Power Combininq Teohnique" by K. J. Russell, published in the IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-27, No. 5, May 1979. This art is directed to the use of identical signal handling devices arranged in symmetrical structures, so that the signals arriving at the combining point are in-phase. In other cases, as in U.S. Pat. No. 4,291,278 issued Sept. 22, 1981 to Quine, the structure is not symmetrical, and phase shifters are included in order to compensate for phase differences produced by the asymmetrical structure.
It is desirable to improve the bandwidth of parallel signal amplifiers.